blender grease pencil / Krita workflow question
To open a grease pencil animation in Krita, you first render the animation as a OpenGL animation. This will give you a sequence of images. Then you open Krita and start a new project with the same dimensions(or not) as your animation. Then you select File>Import animation frames and select the image sequence. 

As for the painting, the author painted on a separate layer and used the Transform Layer brush with the Liquify option to manipulate the painted layer to stay with the animation. There is also an auto-frame option to keep changes per frame.